A typical optical spectrograph includes a small input aperture, typically a slit, however, can alternatively be a circular pinhole or an optical fiber; however, for the sake of brevity, will hereinafter be referred to as a slit. A converging cone of light, is projected towards the slit and a portion of the light passes through the slit. In a typical optical spectrograph, this slit of light is projected onto a lens which collimates the slit of light to form a beam of parallel light rays. In a typical optical spectrograph, a dispersive element, such as, a prism, a transmission grating, or reflection grating, bends the collimated beams by differing amounts, depending on the wavelength of the light. Typically, a camera lens brings these bent collimated beams into focus onto an array detector, such as, a charged-coupled device (CCD) detector located at the final focal plane, and which may record the light intensities of the various wavelengths.
In a typical optical spectrograph, the collimating lens and the camera lens act as an image relay, to create images of the light passing through the slit on the detector, such as a CCD detector, which may be displaced laterally depending on the wavelength of the light. The resolution of an optical spectrograph, i.e., its ability to detect and measure narrow spectral features such as absorption or emission lines, can be dependent upon various characteristics. Such characteristics may include the dispersing element, such as, the prism, transmission grating, or reflection grating; the focal length of the camera lens; and the width of the slit. For a particular disperser and camera lens, the resolution of the spectrograph can be increased by narrowing the width of the input slit, which causes each image of the light passing through the slit (depending on the wavelength of the light) and onto a detector, subtending a smaller section of the detector, allowing adjacent spectral elements to be more easily distinguished from each other.
By narrowing the width of the input slit, less light passes therethrough, which can reduce the quality of any measurements due to a reduction in the signal-to-noise ratio. In some applications, such as astronomical spectroscopy, high-speed biomedical spectroscopy, high-resolution spectroscopy, or Raman spectroscopy, this loss of efficiency can be a limiting factor in the performance of the optical spectrograph. A device which increases the amount of light that can pass through the slit by horizontally compressing and vertically expanding a spot image of an input beam of light, producing a slit, while substantially maintaining light intensity or flux density, would be advantageous in the field of optical spectrography.
A person of skill will understand that the terms horizontal, vertical and other such terms used throughout this description, such as, above and below, are used for the sake of explaining various embodiments of the invention, and that such terms are not intended to be limiting of the present invention.
Optical slicers can be useful to receive an input beam and produce output beams for generating slits. The use of transparent prisms and plates to slice an input beam can produce a slit that is tilted along the optical axis, and additionally the slicing of an optical beam can occur along the hypotenuse of a 45° prism, which can result in focal point degradation due to different sections of the sliced image being located at different focal positions. The performance of such slicers can depend on the absorption coefficient and index of refraction of the prism used (both wavelength dependent). These deficiencies can limit the use of such slicers as broadband devices.
Other slicers, such as pupil slicers, possess drawbacks such as the inability to obtain high-resolution spectral information from different portions of an image. Additionally, such slicers can be large in size, and can result in reduced or inefficient implementation with a variety of systems. Current slicers that employ a glass-based design tend to use a Lagrange-constant transformer to bring light from a Raman optical source to an optical spectrometer. The transformer involves eight different cylindrical and spherical lenses, as well as two stacks of ten precisely positioned cylindrical lenses. The resulting device can have a length of more than 58 inches along the main optical axis, a size at which it tends to be both difficult to maintain alignment, and difficult to maneuver or employ in any setting outside of a tightly-controlled laboratory.
In some pupil slicers, two slit images can be generated on different portions of a CCD detector. This implementation can present the disadvantage that the slit images are spaced on the detector with gaps in between, which can add noise to the signal, decreasing the quality of the output data. Additionally, in such slicers, the gaps can waste valuable detector area, limiting the number of spectra (or spectral orders) that can be fit upon the detector. Further, when using such slicers, the detector readout may not be optimal due to the spectrum being spread over the detector area.
Slicers using optical fiber bundles to allow the extended (often round) image of an input source to be formed into a narrow slit can cause the degradation of the output ratio to be large and the total performance to be inefficient. Existing slicer devices uniformly suffer this decreased efficiency and output ratio, representing a clearly-defined objective of slicer design and implementation.